The present invention concerns a new muffler which can be used, in particular, for the catalytic purification of the gases produced by an internal combustion engine.
It is known to remove the polluting products such as unburnt hydrocarbons, nitrogen oxides and carbon etc... contained in the exhaust gases of an internal combustion engine by contacting said gases with a catalyst, formed of particles (balls or extrudates for example), which favours the reduction or oxidation reactions of the polluting products, at temperatures lower than that necessary to achieve the complete removal of said polluting products, in the absence of catalyst.
Catalytic mufflers are, for example, described in French Pat. Nos. 2,373,677, 2,198,536, and 1,299,792, as well as in the German Pat. No. 2,310,843.
One of the problems encountered when using these prior art mufflers is that of the formation of voids within the catalyst bed after a more or less long period of time in use.
As a matter of fact, it is not possible to avoid the decrease of the catalyst volume introduced into a muffler as time lapses: the main reasons are the lack of packing of the catalyst, the thermal skrinkage of the particles of which it is formed, the breaking of certain grains more brittle than the others, etc... In the catalytic mufflers with flat bed and downward gas flow (developed by General Motors in U.S.A. for example) this volume decrease is not very serious since the catalyst is compressed by its own weight, and by the effect of the gas velocity against the lower grid of the catalytic muffler.
However, every time there is insufficient room available to house a flat bed catalytic muffler, it is necessary to make use of cylindrical mufflers of the "radial flow" type, such as, for example, as that described in the French Pat. No. 7,406,395.
In these mufflers with radial flow a void is generated at the upper part of the muffler where through gases flow upwardly. The balls or grains of catalyst are then lifted by the gas flow and their motion results in a wear of these particles which may be very rapid.
It is already known, from U.S. Pat. No. 3,594,131, to make use of a catalytic muffler for purifying the exhaust gases of an internal combustion engine, with said muffler comprising an elongate body including an admission gas chamber and an exhaust gas chamber, provided with ports substantially placed at the respective two ends of the muffler, an elongate annular space adapted to contain a purification catalyst formed of particles placed inside said elongate body and separating said gas admission and exhaust chambers. The space is defined by walls which comprise two lateral walls provided with perforations, at least one of the walls defining said space being mounted so as to be slidable in the direction of the length of the muffler. In addition there is included means for pressing the catalyst by action on said sliding wall, which tends to reduce the volume of said space adapted to contain the catalyst.
The advantage of this type of muffler is to maintain the catalyst compressed, and to avoid the formation of voids within the catalyst bed.
However, in the device of the type described in the U.S. Pat. No. 3,594,131, compression of the catalyst particles results either from the action of the gravity acting on a horizontal wall, (embodiment illustrated in FIG. 1 of this prior patent), or from the action of resilient means such as a spring, but said spring is then in contact with the hot exhaust gases, which is likely to result in a deterioration of the mechanical properties of the spring (embodiment illustrated in FIG. 2 of U.S. Pat. No. 3,594,131).